1. Field of the Invention
The present invention relates to a novel aminopeptidase and a method of hydrolyzing a peptide or protein by use of the aminopeptidase or a cell extract containing the aminopeptidase.
2. Discussion of the Background
An aminopeptidase is an enzyme which releases an N-terminal amino acid sequentially from a peptide or protein.
The decomposition of proteins such as soybean protein, wheat gluten, casein etc., particularly the decomposition of soybean protein into amino acids has been carried out conventionally using acid hydrolysis with hydrochloric acid or sulfuric acid or the conventional proteases derived from microorganisms, e.g. aspergillus (Japanese Patent laid-open Publication No. 70,852/1976, Japanese Patent Publication No. 32,344/1980, Japanese Patent Publication No. 60,480/1991, Japanese Patent laid-open Publication No. 239,966/1987, Japanese Patent laid-open Publication No. 2392/1990 and Japanese Patent laid-open Publication No. 112,461/1991).
The preparation of a soybean protein hydrolysate as a candidate for a natural seasoning by hydrolyzing soybean protein with an acid requires a reaction at 100.degree. C. for 1 to 2 days, and at such a high-temperature and long reaction time requires high energy consumption. On the other hand, the acid hydrolysis of a protein is easy, but there are problems in that the resulting amino acids are also decomposed (destroyed) and a high salt content results from neutralization.
An approach to these problems was to decompose a protein under mild conditions by the use of a conventional protease. However, the conventional proteases, typically papain, subtilisin etc. are endopeptidases, and thus they decompose a protein into peptides, but only slightly decompose the peptides further into amino acids. Therefore, aspartic acid and glutamic acid participating strongly in taste were only slightly released, while bitter tastes were brought about, and the resulting hydrolysate cannot be used as a seasoning liquid.
To solve this problem, the combined use of exopeptidases, i.e. a group of enzymes decomposing a peptide into amino acids, such as aminopeptidase, carboxypeptidase etc., is considered effective. For example, the importance of leucine aminopeptidase and acid carboxypeptidase is mentioned to increase the content of free amino acids for decomposing soybean protein with aspergillus, typically for making soy sauce by fermentation (Tadanobu Nakadai, Shouken, Vol. 11, No. 2, (1985)).
In this literature, however it is also described that in soy sauce there still remain dipeptides and tripeptides containing acidic amino acids in their sequences and these peptides are only slightly decomposed with a peptidase derived from aspergillus. Further, these dipeptides and tripeptides also include a large number of peptides containing glutamic acid or aspartic acid at the N-terminal end.
The term "peptides only slightly decomposed" as used herein means that an enzyme serving as the catalyst for decomposition, that is, a peptidase, has low substrate specificity for them.
This poor ability to decompose dipeptides and tripeptides containing acidic amino acids in their sequences is not only a problem with the peptidase derived from aspergillus in making soy sauce by fermentation, but also a problem with commercial peptidase preparations, typically those derived from aspergillus.
In the soy sauce industry etc., therefore, there is a demand for the discovery of a peptidase which effectively decomposes low-molecular-weight peptides containing glutamic acid and aspartic acid in their sequences in order to raise the degree of released amino acids in a peptide or protein hydrolysate.